DEPARTMENT OF COMMERCE 



Scientific Papers 



OP THB 



Bureau of Standards 

3. W. STRATTON. Director 

No. 387 
PERMEABILITY OF RUBBER TO GASES 



BY 



JUNIUS DAVID EDWARDS, Associate Chemist 
S. F. PICKERING, Associate Chemist 
Bureau of Standards 



JULY 12, 1920 




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Scientific Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 387 
PERMEABILITY OF RUBBER TO GASES 



BY 



JUNIUS DAVID EDWARDS, Associate Chemist 
S. F. PICKERING, Associate Chemist 
' Bureau of Standards 



JULY 12, 1920 




PRICE. 10 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 

Washington. D. C. 

WASHINGTON 
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n; of D, 

SEP 23 1920 



p 



y 



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o7(J^ 



PERMEABILITY OF RUBBER TO GASES 



By Junius David ELdwards and S. F. Pickering 



CONTENTS 

Page 

I. Introduction 327 

II. Nature of permeability process 328 

III. Methods of determining permeability and characteristics of rubber sam- 

ples employed 328 

1. Methods 329 

2. Characteristics of rubber samples employed 330 

IV. Relatioi^sf permeability to composition of rubber 332 

V. Relation of permeability to experimental conditions 338 

1. Relation of permeability to pressure 338 

2. Relation of thickness of rubber to permeability 342 

3. Time of penetration of rubber 344. 

4. Relation of permeability to temperature 345 

VI. Permeability of rubber to various gases 347 

1. Permeability of rubber to hydrogen 347 

2. Permeability of rubber to oxygen 348 

3. Permeability of rubber to nitrogen 349 

4. Permeability of rubber to argon 350 

5. Permeability of rubber to air 350 

6. Permeability of rubber to carbon dioxide 351 

7. Permeability of rubber to helium 352 

8. Permeability of rubber to ammonia 354 

9. Permeability of rubber to ethyl chloride 356 

10. Permeability of rubber to methyl chloride 356 

11. Permeability of rubber to water vapor 357 

VII. Theory of permeability 360 

VIII. Summary 361 

I. INTRODUCTION 

Rubber has been in everyday use as a gas-retaining material for 
a great many years. Nevertheless, until the recent development 
of the modem rubberized balloon fabric, comparatively little 
advance was made in oiu- knowledge of the permeability of rubber 
to gases. With the development of fabrics for lighter-than-air 
craft came the demand for accurate methods of measuring per- 
meability, together with a demand for the most varied kinds of 
information regarding the permeability relations of rubber and 
gases. The Bureau of Standards has already, in its Technologic 

327 



328 Scientific Papers of the Bureau of Standards [Vot.i6 

Paper No. 113/ published the results of an investigation of methods 
for the determination of the permeability of rubber to hydrogen. 
The present investigation of the factors involved in the passage of 
gas through rubber and the permeability of rubber to different 
gases has been correlated with that work. The experimental 
work extended from 1917 to 1919; its publication has been de- 
layed for obvious reasons. 

II. NATURE OF PERMEABILITY PROCESS 

Graham,^ in his work on the " Dialytic Separation of Gases by 
Colloid Septa," was the first to point out that the characteristic 
passage of gas through rubber took place by solution in the rubber 
and not by diffusion through microscopic openings. If gases 
passed through rubber by the process of diffusion, as through a 
porous plate, their rates of penetration should be approximately 
inversely proportional to their viscosities. As pointed out by 
Graham, the relative rates of penetration of different gases bear 
no relation to their densities or viscosities. In fact, it is difficuit 
to correlate the permeability with any of the well-known proper- 
ties of the gases. It is quite obvious from a consideration of the 
facts that some phenomenon other than that of diffusion through 
small openings is concerned and that the properties of both 
rubber and gas determine the rate of penetration. Before enter- 
ing on a discussion of this point the experimental facts which 
bear on the case will be presented. 

III. METHODS OF DETERMINING PERMEABILITY AND 
CHARACTERISTICS OF RUBBER SAMPLES EMPLOYED 

The permeability of a rubber film may be defined as the rate at 
which it is penetrated by a certain gas. Permeability will be 
expressed in terms of liters of gas per square meter per 24 hours, 
the volume of gas being corrected to the standard conditions of 
o°C and 760 mm mercury pressure. Unless stated otherwise, all 
determinations are made under the following conditions, which 
are adopted as standard for this work: The fabric is held at a 
temperature of 25° C, with air at atmospheric presstue (760 mm 
of mercury) on one side of the fabric and the gas in question at an 
excess pressure of 30 nun of water on the other side. 

' J. D. Edwards. Determination of Permeability of Balloon Fabrics, B. S. Tech. Paper No. 113; 191S. 
2 Phil. Mag., 82, p. 401; 1S66. 



Edwards "I 
Pickermgi 



Permeability of Rubber to Gases 
1. METHODS 



329 



Most of the different types of apparatus available for the deter- 
mination of permeability have been described in Technologic 
Paper No. 113, to which reference has been made. Certain other 
apparatus developed recently will be mentioned in connection 
with the experimental work. 

What may be called the standard apparatus of the Bureau of 
Standards is shown in diagram in Fig. 1. The rubber sample to 
be tested is held in the permeability cell a, which is maintained at 
a constant temperature in an air or water bath h. The cell con- 
sists of two circular plates with a shallow chamber in each. The 
test piece is held between the flanges of the cell and separates the 




Fig. I . — Standard apparatus for determining permeability of rubber to gases 

two chambers; it is supported by a series of crossed wires in the 
form of a screen. A constant concentration of the gas whose 
permeability is to be measured is maintained in one chamber. 
The gas which penetrates the exposed area of rubber passes into 
the other chamber, from which it is continuously removed by a 
stream of air or other gas and determined quantitatively. 

Because of the common use of hydrogen in balloons, the per- 
meability to hydrogen is the property most often determined in 
the case of balloon fabrics. For this reason, and because of the 
accuracy with which the permeability to hydrogen can be deter- 
mined, the permeability to any other gas will be referred to its 
permeability to hydrogen as the standard of comparison. 



330 Scientific Papers of the Bureau of Standards [Voi. t6 

In determining the permeability to hydrogen a current of pm-e, 
dry hydrogen is passed over one side of the fabric and out through 
a water seal. Dry air imder carefully regulated pressure is passed 
over the other side of the fabric through a drying tube d^ and into 
one chamber of a gas interferometer, where the percentage of 
hydrogen in the air is determined optically. The gas then passes 
out through the drying tube d^, which prevents diffusion of water 
vapor into the interferometer, through the saturator / filled with 
glass beads partly covered with water, and then through the wet 
meter m. The saturator is employed to prevent loss of water 
from the meter by evaporation into the gas which is being meas- 
ured. Arrangements are made for by-passing the air stream 
from the interferometer to the meter when the interferometer is 
being read and for supplying the comparison chamber of the 
interferometer with pure, dry air. 

The gas interferometer ' of the Rayleigh type measures the 
difference in refractivity of the two samples of gas contained in 
the gas chambers of the instrument. Several interferometers 
were used, and their sensitivity was such that each scale division 
indicated from 0.007 to o.oi per cent hydrogen in air. The average 
of 10 settings of the instrument gave a reading which was good to 
somewhat better than i scale division ; this gives ample precision 
in the determination of the hydrogen. The calibration of the 
interferometer, both for hydrogen and other gases, was accom- 
plished by the method described by one of the present authors in 
the Journal of the American Chemical Society.* By the use of this 
method the utility of the interferometer was greatly extended, 
and we were enabled to handle acciurately such mixtures as helium 
and air, which are difficult to analyze by other methods. The 
interferometer furnishes a rapid and accurate means of analyzing 
many gas mixtiu-es, and its use will be discussed further in that 
connection in this paper. 

2. CHARACTERISTICS OF RUBBER SAMPLES EMPLOYED 

The greater part of the determinations recorded were made with 
rubber films as they are contained in balloon fabrics. This was 
done not only because of the immediate application of the results 
in that field, but also because balloon fabrics of great variety were 
readily available. Rubber films of satisfactory imiformity and 
low permeability are also most easily secured in the form of balloon 

• For detailed description see L. H. Adams, J. Am. Chem. Sec., 8J, p. iiSi; 1915- 
' Edwards, J. Am. Chem. Soc„ 39, p. 3383: 19x7. 



^"fng] Permeability of Rubber to Gases 331 

fabrics. The support given the rubber film by the cloth on which 
it is spread simpUfies the handling and testing of the material. 
The question might be raised as to whether in some cases the 
results might not be influenced by the cloth on which the rubber 
is spread. To test this point, determinations were also made on 
thin sheet rubber in those instances. The absolute permeability 
of the rubber is profotmdly modified by the cloth, as will be shown 
later ; its relative permeability to different gases is apparently not 
affected thereby. 

The presence of the cloth, however, introduces a factor which 
may lead to serious errors in testing if not properly taken account 
of. Most balloon fabrics are constructed of two plies of cloth 
with a film of rubber between the plies and a thinner coating of 
rubber on the inside and outside for the purpose of protection; 
these inner and outer coatings have little effect in reducing the 
permeability of the fabric. The rubber does not penetrate very 
thoroughly into the interstices between the threads, and, as a 
result, hydrogen is able to diffuse laterally along the cloth as 
well as directly through the rubber film. Hydrogen can there- 
fore diffuse along the textile and into the area clamped between 
the edges of the cell which, it might be assumed, is not active in 
the test. Here it can pass through the main layer of rubber, 
back through the textile on the other side, and into the air 
chamber. The exposed or "active" area of fabric is, then, larger 
than the area defined by the edges of the cell, and the results are 
correspondingly high. If there be no rubber on either side or 
only on one side, the interstices in the cloth can be satisfactorily 
sealed with vaseline or soft wax applied hot, which fills up the 
openings between the threads and prevents lateral diffusion of 
the hydrogen. If the fabric has a rubber coating on both sides, 
the vaseline can not penetrate this rubber into the cloth under- 
neath ; no satisfactory method of sealing such fabrics is available. 
The best procedure in that case, is to reduce the margin of the 
fabric to as small an area as possible and put hot wax on the edge. 
The possible error, if the whole margin is active, can then be 
estimated. 

The " edge effect" can be illustrated by the results of a series of 
experiments on limiting the area of a test piece (see Table i). 
Two samples of two-ply fabric were tested, one having an outside 
rubber coating on one side only and the other being rubber coated 
on both sides. The total area of each test piece was about 130 



332 



Scientific Papers of the Bureau of Standards 



\Vol. i6 



cm% but the exposed area was reduced to loo, 90, and 70 cm ^ by 
coating with grease. With the fabric having one cloth surface 
it is seen that the area is accurately defined in each test. With 
the fabric having rubber on both surfaces, practically the whole 
area of the test piece is effective. 

TABLE 1.— Effect of Limiting Area of Test Piece 





Apparent permeability— Liters; eiposed 
area<2 limited to — 




100 cm« 


90 cm2 


70 cm» 


Rubber coat on one side only of two-ply fabric 


12.1 

11.8 


12.2 
15.8 


12.1 




16.6 







oThe exposed area was used in calculating the permeability per square meter. 

The fabric (No. 50313) with which a great deal of the experi- 
mental data were obtained in succeeding experiments was a two- 
ply fabric without rubber coating on either side. Where it was 
necessary for some reason or other to use fabrics having rubber 
on both sides, the edge effect was made as small as possible by 
reducing the margin to a minimum ; its effect on the relative values 
of tests was then without significance. 

IV. RELATION OF PERMEABILITY TO COMPOSITION OF 

RUBBER 

Crude rubber, as well as vulcanized rubber, may vary so widely 
in composition and physical characteristics that one can hardly 
expect to find or define such a constant as the specific permeability 
of rubber. Part of the disagreement between previous experi- 
menters has been ascribed to differences in the samples of rubber 
which were tested. Nevertheless, certain regularity of behavior 
has been noted and certain observations made on the relation 
between permeability and composition which are of interest and 
value. 

For the present purpose rubber may be considered to be a 
mixture of "polyprene" (CsHg)! in different stages of polymeriza- 
tion, together with resins, nitrogenous matter, water, and inorganic 
material in varying proportions. Vulcanized rubber, which we 
will hereinafter refer to simply as rubber, contains, in addition, 
varying proportions of sulphur, combined with or adsorbed by 
the polyprene, together with some free sulphur. Compounding 
materials in great variety may also be added to the rubber to 



Edwards "1 
Pickeringi 



Permeability of Rubber to Gases 



333 



give it desirable characteristics, but where imperviousness to gases 
is desired their use is usually restricted. 

The effect of sulphur upon permeability may be considered in 
connection with the effect of vulcanization, since the two factors 
are interrelated. The effect of different degrees of vulcanization 
or "cure" upon permeability is shown, for one compound, by the 
series of tests given in Table 2. The samples were taken from a 
roll of two-ply balloon fabric, different sections of which had been 
given different degress of vulcanization, as indicated. Except for 
variations in the uniformity of spreading, the temperature and 
time of heating were the only variables. 

TABLE 2.— Effect of Time and Temperature of Vulcanization Upon Permeability a 









Sulphur 


Acetone 

extract 

(sulphur 

free) 


Permea- 
biUty 


Sample No. 


(steam heat) 


Com- 
bined 


Free 


37010 


Hours 
None 

0.5 

1.0 

1.0 

1.25 

1.0 

1.5 


°F 


Per cent 

0.3 
.5 
1.4 
1.3 
1.6 
2.5 
2.3 


Per cent 

4.3 
3.2 
2.5 
1.8 
2.1 
2.1 
1.8 


3.8 
3.2 
3.2 
3.0 
3.0 
3.2 
3.0 


Liters 

12.8 


37009 


270 
270 
284 
284 
288 
284 


11.6 




11.5 


37005 


12.7 


37007 ... 


15.5 


37004 


12.2 


37006 


12.8 







a The chemical analysis was made about eight months after the permeability determinations, 
free sulphur may, therefore, be somewhat lower than that originally present. 



The 



This series of fabrics shows no significant variation in permea- 
bility which can not be ascribed to lack of complete uniformity in 
the fabric. The combined sulphur varied from practically 
nothing to 2.5 per cent. 

In a similar series of tests samples were taken from adjacent 
portions of 13 different rolls of fabric before and after vulcaniza- 
tion. In two cases the permeability was the same before and 
after vulcanization; in five cases the permeability of the uncured 
sample was the highest and in the remaining six cases the reverse 
was true. The average difference was only i liter. The average 
combined sulphur before and after vulcanization was 0.76 and i .07 
per cent, respectively ; similarly the average acetone extract was 
3.8 and 3.7 per cent. 

In Table 3 are shown the results of another series of tests in 
which the time and temperature of vulcanization were varied. 
The permeability and chemical characteristics are given both for 

181118°— 20 2 



334 



Scientific Papers of the Bureau of Standards 



[Vol. i6 



the fabric as received and after storage under ideal conditions (in 
a cool, dark place) for 1 2 months. It may be remarked, to begin 

TABLE 3. — Relation of Composition to Permeability "^ 







Composition and permeability 
as received 


Composition and permeability 
alter storage (12 montlis) 


Sample No. 


temperature 
of cure 


Acetone 

eitract 


Free 
sulpiiur 


Com- 
bined 
sulpiiur 


Per- 

mea- 
bUity 


Acetone 
extract 


Free 
sulphur 


Com- 
bined 
sulphur 


Per- 
mea- 
bility 


27003 


Hours 

3 
4 
3 
4 

3 
4 
3 
4 

4 
4 

4 
4 


°F 
270 
270 
290 
290 

270 
270 
290 
290 

270 
290 

270 
290 


Percent 

3.1 
5.1 
5.2 
5.3 

6.1 
5.7 
5.5 
5.8 

5.8 
6.9 

5.3 
6.3 


Percent 

2,9 

3.0 

.9 

1.1 

5.7 
4.4 
1.7 
1.9 

4.9 
1.6 

3.8 

1.7 


Percent 

1.6 
1.9 
3.7 
4.0 

1.8 
2.0 
3.6 
4.8 

2.1 
4.6 

1.7 
5.0 


Liters 
23.5 
20.4 
18.8 
15.8 

20.6 
19.9 
16.8 
14.9 

23.8 
15.9 

20.5 
10.6 


Percent 

5.4 
6.0 
6.3 
7.6 

5.9 
4.2 
12.0 
16.8 

5.3 
13.6 

16.6 
17.7 


Percent 

2.2 

3.2 

.7 

.6 

3.6 

•1.6 

.6 

.5 

3.5 
.5 

.4 
.5 


Percent 

1.6 
2.6 
4.1 

4.7 

3.6 
2.9 
5.2 
6.3 

2.8 
5.7 

6.8 
8.0 


Liters 

17.7 




18.8 


27002 


13.2 




9.5 


26998 


15.4 


26997 


14.9 


26999 


4.0 


26996 


6.0 


26995 


14.0 


26994 


2.4 


26992 


6.8 


26993 


1.3 







a All fabrics were two-ply construction. They varied in weight and distribution of rubber compound. 
The percentage of sulphur was varied in two different proportions, but this was the only change in the 
composition of the rubber compound. Fabrics grouped together were of identical construction except 
for variations in cure. The analyses were calculated on the basis of the rubber compound contained and 
not on the weight of rubber plus fabric. 

with, that practically all of these fabrics were somewhat over- 
cured. The rubber had the characteristic odor of overcured 
balloon fabric, and many of the samples became quite stiff with 
time ; some reached the stage where the rubber compound was 
brittle and cracked on bending. The most noticeable facts which 
these results show are that with these fabrics the permeability 
decreased during storage and that there was a concomitant increase 
of combined sulphur and acetone extract. If the percentages of 
combined sulphiur are plotted as abscissae and the permeabilities 
as ordinates, the graph shown in Fig. 2 is obtained. As shown 
by the legend, data on fabrics which have been exposed outdoors 
for 30 days are also included. There apparently is some relation 
in this series between the permeability and the percentage of 
combined sulphur. The acetone extract also increases at the 
same time, but there is no such striking relation between these 
two variables as that shown in Fig. 2. The original acetone 
extract is about the same on all the fabrics, but the permeability 
shows a considerable variation. 



Edwards 1 



Permeability of Rubber to Gases 



335 



Similar decreases in permeability are observed when fabrics are 
exposed to the weather. In Fig. 3 are shown the relations between 
permeability and period of exposure for three different fabrics. 
The periods of testing were not frequent enough to locate the lowest 
point on each curve, but the curves indicate that the permeability 
reaches very low values. This lower permeability is accompanied 



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Fig. 2. — Graph showing decrease in permeability of rubber with increase 
of combined sulphur 

by a characteristic hardening of the rubber. When the rubber film 
becomes sufficiently brittle it cracks easily and thereafter shows a 
very high permeability. The changes in permeability shown by 
the graphs were accompanied by the following changes in chemical 
characteristics at the time the lowest permeability was observed. 



336 Scientific Papers of the Bureau of Standards ivoi.16 

TABLE 4.— Change in Chemical Chaiacteristics of Rubber in Three Fabrics of Fig. 3 





Fabric No. 10650 


Fabric No. 10652 


Fabric No. 23990 


Composition 


Original 


45 days 


Original 


60 days 


Original 


150 days 




Per cent 

1.4 

.5 

3.1 


Per cent 

1.7 

.2 

50.0 


Per cent 
3.0 
2.8 
3.2 


Per cent 

4.2 
.2 

16.2 


Per cent 
1.5 
1.2 
4.0 


Per cent 
2.5 




.2 




42.9 







In the case of fabrics exposed outdoors there is sometimes an 
increase in permeability during the initial period of exposure, 
which is followed later by the customary decrease. This increase 
is not accompanied, apparently, by any significant change in 
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Time of £xpoaure -aay<s 

Fig. 3. — Egect upon permeability of exposure of rubber to the weather 

Preliminary to drawing any conclusions from them it may be 
desirable to summarize the observed facts in regard to permea- 
bility and composition as shown by the preceding tests. 

1. The aging of rubber in thin films is accompanied by a char- 
acteristic decrease in permeability. 

2. The aging of rubber is usually accompanied by a decrease 
in the percentage of total sulphur; the combined sulphur increases 
by varying amoxmts and the free sulphur decreases eventually to 
a low value. 



Scientific Papers of the Bureau of Standards, Vol. 16 




Fig. 4. — Section of balloon fabric, showing crystals of sulphur. (XjjS) 



pil"ins] Permeability of Rubber to Gases 337 

3. In one series of fabrics where the degree of cure was varied 
no significant change in permeability was observed. In this case 
the percentage of combined sulphmr varied from 0.3 to 2.5 per 
cent. In another series large changes in permeability were noted 
with change in the degree of cure; the combined sulphur varied 
from 1.6 to 5 per cent. The original acetone extract was approxi- 
mately constant in each series. 

Because of the number of factors involved and because of the 
relatively small number of data presented, it would be imwise to 
draw any very extensive conclusions. The view is quite widely 
held by manufactvu-ers and others that the permeability of a 
fabric can be reduced by increasing the degree of cvu"e. Between 
certain limits this is true. That this reduction in permeability is 
caused entirely by the increase in combined sulphiu: is not at all 
certain. Opposed to this latter view is the fact that as great and 
greater decreases in perxneabiUty are noted on exposed fabrics 
where there are relatively small changes in combined sulphur. 
(See Fig. 3 and Table 4.) The most striking change in exposed 
fabrics is the increase in acetone extract, which increase is a meas- 
ure of the resinification and oxidation of the rubber. It appears 
reasonable to believe, therefore, that an increase in both the com- 
bined sulphur and acetone extract causes a decrease in permea- 
bility. This would be the natural result if hydrogen was insoluble 
in both the acetone-soluble material and the " polyprene sulphide." 

It has been thought by some that the free sulphur plays an 
important part in determining the permeability. The free sulphiu: 
which is present in the colloidal condition in the rubber after vul- 
canization frequently crystallizes out. This is strikingly shown 
by the microsection of a sample of ballonet fabric illustrated in 
Fig. 4. The sulphtu- crystals are seen as dark dendritic masses in 
the rubber between the two plies of cloth. A certain amoimt of 
this sulphur eventually penetrates to the surface and evaporates. 
This process might possibly produce a certain porosity which 
would increase the permeability. Tests made on portions of the 
fabric of Fig. 4, where crystallization was extensive, showed no 
significant difference in permeability as compared with portions 
where crystallization had not occurred. Certainly our tests and 
experimental methods have not been of sufficient deUcacy to 
detect any effect on the permeability which can be ascribed to 
this blooming out of sulphur. 



338 Scientific Papers of the Bureau of Standards [Voi. ns 

Compounding materials may be added to the rubber either to 
make it more impervious to gases or to give it greater durability. 
Paraffin and glue are two substances which are said to lower the 
permeability of rubber to hydrogen. It is known that either of 
them alone will give a film of very low permeability, provided it 
is nonporous. Their use is not essential, however, to the produc- 
tion of a satisfactory coating for balloon fabrics. Lampblack, 
zinc oxide, or litharge may be incorporated in the rubber to give it 
greater life by protecting the rubber from the injurious action of 
light. No systematic investigation of the effect of these sub- 
stances has been tmdertaken; otir work has been confined almost 
exclusively to rubber compounds of the simplest composition. 

V. RELATION OF PERMEABILITY TO EXPERIMENTAL 

CONDITIONS 

1. RELATION OF PERMEABILITY TO PRESSURE 

In considering the effect of pressine, a distinction should be 
made between the total pressure and the partial pressure of any 
constituent. A difference in the total pressure on the two sides 
will produce tension in the rubber film and a change in thickness 
or physical properties may result. The effect of a change in the 
total pressure will be influenced by the support given the rubber 
film, such as when it is held between cloth of one or more plies, as 
in the case of a balloon fabric. 

The work of previous investigators indicates that the perme- 
ability of rubber to any gas is about proportional to the partial 
pressure of that gas. The agreement on this point is not unani- 
mous, however, and the methods and data recorded are not 
satisfactory in all particulars. Almost all of the recorded meas- 
urements were made with apparatus of the volume-loss type; 
that is, they measured the loss in voltune of a mass of gas con- 
fined by the rubber. The diffusion took place under varying 
pressure, and the back diffusion of air was seldom corrected for. 
In some cases the total pressure and partial pressm-e varied 
simultaneously, a condition which is obviously imdesirable. 

In Fig. 5 is shown the relation between permeability and 
difference in partial pressure of hydrogen, as shown by tests on six 
different test pieces of the same fabric. The percentages of 
hydrogen in the air were determined by means of the interfer- 
rometer. The permeability of the different test pieces with loo 
per cent hydrogen varied from 9.4 to 10 liters; each result was, 



Pickerin^i 



Permeability of Rubber to Gases 



339 



therefore, multiplied by the ratio of lo to the observed permea- 
bility at ICO per cent, so that the loo per cent value became lo 
in each case and all the other values were directly comparable. 
It may be concluded from these results that the permeability is 
directly proportional to the partial pressure, within the limits of 
experimental error. 

Similar results were obtained with carbon dioxide, as shown 
in Fig. 6. In addition to the balloon fabric (No. 50313) a sample 
of thin rubber known as "dental dam" was also tested. The 
permeability in each case was directly proportional to the partial 
presstue, any deviation being reasonably ascribed to experimental 
error. The values indicated for fabric No. 50313 are the averages 
of tests on seven different test pieces. To make the results 

/z 



h 

r 



/o zo 30 -to so 60 70 ao 90 /00% 
Mydrogen in <air -Par cent 

Fig. 5. — Relation between permeability and partial pressure of hydrogen 

directly comparable and save plotting a graph for each of the 
seven test pieces, the results were corrected, as in the case of the 
experiments with hydrogen, by reducing the 100 per cent values 
to the same figvue by direct proportion and changing the other 
values proportionally. 

In accordance with the conclusion that the permeability was 
directly proportional to the partial pressure, the results of all 
permeability determinations have been corrected to the standard 
condition of a partial pressure of 760 mm in the following man- 
ner: In one determination with carbon dioxide (99.9 per cent 
ptu-e) there was 0.6 per cent carbon dioxide in the air on the 
opposite side of the fabric; the barometric pressiu-e was 750 mm 
and the observed permeability 20 liters. The corrected perme- 






ability is then equal to 20.0 X 



1 00.0 



760, 

750 "99-9 -06 



, or 20.4 liters. 



340 



Scientific Papers of the Bureau of Standards 



[Vol. i6 



The change in permeability when the difference in the total 
pressure on the two sides of the sample is varied follows no simple 
law. In this case, not only does the permeability change with 
the change in partial pressiare, but also it may change with any 
variation in thickness caused by the tension on the rubber. Ob- 
viously, the effect will vary with the support given the rubber. 
In the case of a balloon fabric the rubber film is given very inti- 
mate support by the cloth on which it is spread. The cloth may 
to some extent be prevented from stretching by the manner in 



4i 

40 




















/ 


















/ 


/ 


3Z 

9.; 
















/ 


/ 
















/ 


f 
















/ 


/ 






y 


1 










4 


/ 




yi 


4^ 










</ 


Y 


y 


o/*^ 








1 

6 
4 






/ 


/ 


( 


y 














/ 


X 


y 














A 


y 
















/ 


/ 



















/o 



go so -^ So 60 70 ao &0 
Carbon dioxide m <3ir- Per cent 



/oo 



Fig. 6. — Relation between permeability arid partial pressure of carbon 

dioxide 

which it is held in the cell during a test. In the case of a sheet of 
rubber such as dental dam, the only support the rubber receives 
is from the screen on which it rests in the cell and the fact that 
it is clamped at the edges. In Fig. 7 are shown the graphs of 
several experiments where the pressure of the hydrogen was 
varied. Four different pieces of fabric No. 50313 were tested, 
and, to make the results more nearly comparable in the graph, 
the values were reduced by direct proportion, so that the values 
at 100 mm were identical; the 100 mm point, therefore, repre- 



Edwards T 
Pickering] 



Permeability, of Rubber to Gases 



341 



sents four determinations. The two balloon fabrics (Nos. 50313 
and 47174) show about the same small rate of increase of per- 
meability with increase of pressure. The two samples of dental 
dam show a slightly higher rate. The sample of dental dam show- 
ing the higher permeability was supported in the cell during the 
test by a screen having openings 4 cm^ in area; the other sample 
was supported by wire gauze of 28 mesh. At least part of the 
difference in permeability can be ascribed to the greater stretch- 
ing of the rubber in the case of the first sample mentioned. The 
extensibility of rub'ber and cloth may vary so greatly that no 

/s 
// 



'^,^^ 



/s, 



I 




SoSIS 



Fig. 7. 



/O £0 JO -fO so 6O 70 so 90 /OO /JO 

Prestsure oT hydrogen 
mm of hrdfer 

-Relation between permeability and total pressure oj 
hydrogen 



great imiformity in the snape of these curves can be expected, 
and, in general, that is our experience. However, the rate of 
increase of permeability is much smaller above than below 60 
mm in every case. For the sake of comparison, there is included 
the graph showing the change in permeabilty of a lo-liter fabric, 
which would be caused by the increase in partial pressure of the 
hydrogen; this increase is only o.i liter in the range from o to 100 
mm of water presstire. The slopes of the curves of Fig. 7 are a 
little lower than those shown in Technologic Paper No. 113 
(p. 14) for the same relation. 
181118°— 20 3 



342 



Scientific Papers of the Bureau of Standards 



[Vol. 16 



2. RELATION OF THICKNESS OF RUBBER TO PERMEABILITY 

The permeability of a sample of rubber should obviously bear 
some relation to the thickness of the material. The most reason- 
able assumption, and the one usually made, is that the permeabihty 
is inversely proportional to the thickness of the rubber. A series 
of samples of rubber, identical in chemical composition and physi- 
cal properties, but differing in thickness, was not available for 
testing this point. It was necessary, therefore, to use material 
from different sources and varying in composition. The samples 
tested varied from the thin sheet rubber known as dental dam, 
about 0.2 mm in thickness, up to sheets of 2 mm thickness. Cer- 
tain of the samples were "vapor ctired" with sulphur chloride 
and the rest steam cured in the usual manner. Their chemical 
characteristics are shown in Table 5. The permeability of these 
samples was determined with zero difference of pressure on the 
two sides of the samples in order not to introduce any variation 
in the tests because of stretching of the material. 

TABLE 5. — Chemical Characteristics of Rubber Samples of Fig. 8 



Sample 
No. 



1 
2 
3 
4 
5 
6 
7 
8 
9 
10 



Thick- 
ness 



Mm 
0.18 
.21 
.22 
.25 
.25 
.26 
.48 
.53 
.53 
.60 



Acetone 
extract 



Percent 

2.7 
2.9 
2.8 
3.3 
2.8 
4.1 
2.6 
3.6 
3.1 
2.2 



Com- 
bined 
sulphur 



Percent 

0.8 
2.8 

.8 
1.3 
2.4 

.8 
2.0 
3.2 
2.9 
1.8 



Free 
sulphur 



Per cent 

0.1 

.2 

.1 

.7 
.2 
.3 
3.1 
1.4 
2.0 
.2 



Sample 
No. 



11 
12 
13 
14 
15 
16 
17 
18 
19 
20 



Thick- 
ness 



Mm 

0.66 
.74 
.76 
.81 
.86 
.89 
.90 
1.93 
2.00 
2 20 



Acetone 
extract 



Per tent 
2.7 
2.3 

3.7 
2.8 
2.7 
3.1 
15.4 
3.5 
4.9 
3.4 



Com- 
bined 
sulphur 



Per cent 
2.4 
2.9 
3.8 
2.5 
3.0 
3.5 
4.9 
3.8 
4.2 
2.9 



Free 
sulphur 



Per cent 
2.6 
1.9 
1.7 
2.1 
2.2 
1.4 

.5 
1.1 
2.6 

.2 



Their permeability was determined with a Shakespere per- 
meameter which was ftumshed to us by Prof. Shakespere, of 
Birmingham University. For descriptions of this apparatus and 
method the reader is referred to Reports of the British Advisory 
Committee for Aeronautics for the last two years, which will 
imdoubtedly be made available eventually. The calibration of 
the permeameter has not been established to our entire satis- 
faction; the absolute values it gives are, however, in substantial 
agreement (about 10 per cent) with those obtained by the method 
of the Bureau of Standards. The instrument gives reproducible 
results of good precision for purposes of comparison. 



Edwards 1 
Pickering! 



Permeability of Rubber to Gases 



343 



The results of these tests are shown in Fig. 8, where the gas 
impedance, by which term the reciprocal of the permeability is 
designated, is plotted as a function of the thickness. There is 
clearly a linear relation betAveen the two variables over a consider- 
able range. Such uniformity as was foimd was hardly expected, 
considering the fact that the samples represented the product of 
a number of different makers and made no pretense of being uni- 
form in composition. It will be noted that the very thin samples, 
about 0.2 to 0.3 mm in thickness, show a lower impedance (higher 
permeability) than corresponds to the straight line. This may 
possibly be due to the greater effect of nonuniformity in the very 
thin material. The sample 0.9 mm in thickness (No. 17), which 












































































































































^ 




















(! 


) 


























"S-* 


































-^ 




































^ 


-^ 


















i^J 






















r^ 


^ 






















^J 














^ 




»-^' 








































Jt-^ 






































^ 


^ 









































012 3 * ^ & 7 a ^ /.o // /£ 13 /^ /s Ji 17 /a /s zo ci iZ 

Thickness of rubber-mm 

Fig. 8. — Relation between gas impedance {reciprocal of permeability) and thickyiess of 

rubber 

showed an impedance of over 0.6, was old and stiff, and its high 
impedance was anticipated because of our observations on the 
decrease of permeability with age. This might also be inferred 
from its high acetone extract and combined sulphur values. The 
sample 1.93 mm in thickness contains glue which may accoimt 
for its slightly greater impedance. 

The results of Fig. 8 show sufficient uniformity to warrant 
calculating what may be called the specific permeability of 
rubber. It may be defined in terms analogous to those in which 
such a property as thermal conductivity is defined, by stating 
that it is the volume of gas which passes through imit area of 
a sample of unit thickness in unit time with a difference in par- 
tial presstne of 760 mm of the gas. The centimeter and minute 
can be conveniently used as imits. The specific permeability 



344 Scientific Papers of the Bureau of Standards [Voi. 16 

to hydrogen at 25° C of vulcanized rubber of the character 
described in Table 6, as calculated from the graph of Fig. 8, is 
20.4X10"° cc per minute. The voliune of hydrogen passing 
through a sample of rubber at 25° C can be approximately cal- 
culated from the following equation: 

,^ _ 2oXio'°XAx^ 
d 
where V is the volume in cubic centimeters, A the area in square 
centimeters, t the time in minutes, and d the thickness in centi- 
meters. 

The only other data in the literature which can be compared 
with the above value are those of Kayser.^ The specific per- 
meability to hydrogen at 25° C as calculated from Kayser's 
equations is 27.4X10"° cc per minute, which value is in fair 
agreement with ours. Although care should be taken not to 
place too great reliance on any value for the specific permeability 
of rubber the characteristics of which are unknown, neverthe- 
less, the preceding equation will be fotmd useful in arriving at 
an approximate figure in many cases. A direct permeability 
determination is the only sure method in any case. 

The data of Fig. 8 confirm the very interesting observation 
of Prof. Shakespere that the gas impedance for a given weight 
of rubber is greatly increased by being spread on cloth. For 
example, a sheet of rubber (density, 0.96) having a weight of 120 g 
per square meter (about 3.5 ounces per square yard) will have 
a thickness of 0.13 mm and, according to Fig. 8, a permeability 

of =23.8 liters. This weight of rubber properly spread 

0.042 

on cloth can be made to give a fabric having a permeability of 

about half that value, or 10 to 12 liters. The cloth, therefore, 

performs a very important function in reducing the permeability, 

in addition to giving the rubber support and protection. As 

pointed out by Prof. Shakespere, this fact is of importance in 

securing balloon fabrics of the lowest permeability. 

3. TIME OF PENETRATION OF RUBBER 

For some piuposes, notably for use in gas masks, it is not the 
maximiun permeability which is of most importance, but the 
time required for the gas to penetrate the fabric. A gas-mask 
fabric loses its protective value about as soon as the poison gas 

'Kayser, Wied. Ann., 43, p. s«; 1891- 



Pkk^fng] Permeability of Rubber to Gases 345 

penetrates in appreciable quantities. Permeability determinations 
of the kind considered in the present work are, therefore, of very 
little value in this connection. 

One test, made on a balloon fabric with the Shakespere per- 
meameter, indicated that hydrogen penetrated the fabric in 
less than i minute and the fabric reached its maximum per- 
meability in from i to 2 minutes. These times include the lag 
of the instrument, so that the actual time required to penetrate 
this fabric must be very small. The permeability of this fabric 
was about 8 liters per square meter per 24 hours. Tests made 
with hydrogen sulphide, in which the gas penetrating the rubber 
was detected with lead-acetate solution, showed that rubber films 
of the character used in balloon fabrics were penetrated with 
great rapidity by hydrogen sulphide also. Any considerable time 
required to reach equilibrium in testing may be generally consid- 
ered to be caused by instrumental lag. Sometimes, however, 
there are actual changes in permeability with long-continued 
tests, which make it appear that equilibritun is being reached 
very slowly. 

4. RELATION OF PERMEABILITY TO TEMPERATURE 

Graham first called attention to the large temperature coeffi- 
cient of permeability. The relation between temperature and 
permeability has been examined since then b}' a number of 
investigators, most of whom are in general agreement, although 
Frenzel's '■ results alone indicate that the relation is a linear one. 
However, there are no reliable values which cover a very large 
range of temperatures. 

A special cell was designed for determining the permeability 
at different temperatiures. It was similar to the regular cell, 
except that each half was provided with a water jacket through 
which water could be circulated by means of a pump. By regu- 
lating the temperature of the water the fabric could easily be 
kept at any desired temperature within the range covered. The 
temperature of the fabric itself was measured by means of a 
copper-constantan thermocouple of No. 36 wire whose "hot 
jtmction" was mounted on the fabric before assembling the cell. 

The results of measurements with hydrogen, helium, and car- 
bon dioxide are assembled in Table 7. Because of possible, if 
not probable, changes in the rubber caused by heating or cooling, 

> Frenzel, "Uber die Gasdiirchlasfiigkeit der BallonstoSe." Dnickerei des Elsassichen Textiblattes un 
Gebweiler. 



346 



Scientific Papers of the Bureau of Standards 



[Vol. i6 



it was not considered advisable to use the same sample for each 
test. Therefore, in every test a determination was made at 
25° C for purposes of comparison. 

For graphic comparison the relative permeabilities are shown 
in Fig. 9. These curves are plotted from the data of Table 6, 

/so 

no 

/60 

/So 
/H> 
/3e 

\ 

I 
^70 

60 

So 
fo 

30 
Zo 
/o 



■rm o /o zo 30 ■K? JO 60 70 80 so /oo 
lemperafure - 'C 

Fig. g. — Relation between permeability and temperature for car- 
bon dioxide, hydrogen, and helium 

but the values have been adjusted proportionally to correspond 
to the same constant value at 25° C. The ratios of the per- 
meabilities at 25° C have been chosen to correspond to the values 
indicated in later sections of this paper. 























1 


1 - 






















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r 






















/ 






















1 


1 






















/ 






















/ 


J 






















/ 






















/ 


/ 






















/ 








% 














/ 










/ 














/ 








/ 














/ 








/ 


/ 


% 


/ 










/ 






/ 


/ 


/ 


> 










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y 


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Edwards 1 
Pickerinffj 



Permeability of Rubber to Gases 



347 



TABLE 6. — Variation of Permeability with Temperature 

A. PERMEABILITY TO CAItBON DIOXIDE 



Sample No. 



Penne- 
ability 
at 25° C 



Liters 
perm! 
per 24 
botus 

26.7 
26.4 
28.0 
27.5 
29.0 



Tempera- 
ture of 
test 



t° 



0.8 
55.0 
55.0 
55.0 
80.0 



Perme- 
ability 
att° 



Liters 
per m^ 
per 24 
hours 

8.56 

72.9 

74.2 

73.1 

115.5 



Sample No. 



Perme- 
ability 
at 25° C 



Liters 
per m^ 
per 24 
hours 

26.7 
28.6 
27.4 
27.4 



Tempera- 

ttrre of 

test 



t° 

80.0 
98.3 
98.5 
98.8 



Perme- 
ability 
att° 



Liters 

per m- 

per 24 

hours 
113.3 
146.2 
142.4 
145.4 



B. PERMEABILITY TO HYDROGEN 



9.30 
9.86 
10.45 
11.50 



1.9 

2.8 
70.0 
70.0 



3.51 
3.79 
44.8 
46.1 



11.53 
11.46 
10.67 



90.0 
90.8 
97.8 



68.5 
69.2 
79.9 



C. PERMEABILITY TO HELIUM 



1 


6.25 
5.76 
6.34 
6.05 


0.6 

.8 

70.0 

70.0 


2.29 
2.37 
24.2 
24.1 


5 


6.51 
6.43 
6.62 
6.48 


90.0 
90.0 
97.3 
97.6 


37 


2 


6 


38 5 


3.. . 


7 


44 6 


4 


8 


44 4 









It will be noted that the curves for hydrogen and helium show 
about the same relative increase with temperature. The change 
of permeability to carbon dioxide, however, becomes practically 
linear after 30° C. No simple relation between permeability and 
temperature has been discovered. Although it is without physical 
significance, the following equation represents the variation of 
hydrogen permeability very closely for this sample : 

Permeability = (38oo + 2i7^-|-2.4<--ho.o38<^) lo-^ 

VI. PERMEABILITY OF RUBBER TO VARIOUS GASES 

1. PERMEABILITY OF RUBBER TO HYDROGEN 

In determining the permeability of rubber to different gases 
it is preferable to refer the values to some standard rather than 
attempt to express the penneability in absolute units. In this 
work the permeability to hydrogen has been adopted as the 
standard rate, because hydrogen is so generally used for filling 
balloon envelopes and because the greatest part of our knowledge 
of the permeability of rubber to gases concerns hydrogen. Accord- 



348 Scientific Papers of the Bureau of Standards [Voi. i6 

ingly, the permeability to hydrogen of any sample of rubber has 
been set equal to unity ; its permeability to any other gas is given 
as the ratio of its permeability to that gas to its permeability to 
hydrogen. In order to secure the required precision, it has been 
necessary to determine the permeability of every test piece both 
to hydrogen and to the gas in question. 

The hydrogen used in all this work was made in a Kipp generator 
from a very pure lot of zinc and from "C. P. " hydrochloric acid. 
It was purified by passage over soda lime and anhydrous granular 
calcium chloride. Conclusive tests made in another connection 
on hydrogen generated in this way showed it to contain not more 
than 3 or 4 parts in lo coo of impurity when the generator was 
properly swept out.' 

It was foimd that the ratio of permeabilities for different gases 
with different samples of rubber was fairly constant — sufficiently 
so to make the results of interest and value. In all probability 
the ratio varies somewhat with different samples of rubber; the 
extent of this variation is indicated roughly b}' the concordance 
of the results secured with different samples. In a preceding 
section a first approximation to the specific permeability of 
rubber to hydrogen was given ; this value multiplied by the ratio 
of permeabilities of different gases will give approximately the 
specific permeabilities of those gases. 

2. PERMEABILITY OF RUBBER TO OXYGEN 

It is an interesting fact, first pointed out by Graham, that rub- 
ber is more permeable to oxygen than to nitrogen. As a result, 
air which has passed through rubber contains a higher percentage 
of oxygen than normal air. The significance of this fact in con- 
nection with the use of rubber-coated balloon fabrics has already 
been discussed in a report from this laboratory.^ 

The permeability of rubber to oxygen was determined with the 
apparatus of Fig. i, with some appropriate minor changes. Air 
can not well be used as the comparison gas in the interferometer 
because its refractivity is not sufficiently different from that of 
oxygen, which fact gives rather low sensitivity. Hydrogen, how- 
ever, differs greatly from air in refractivity and is therefore well 
adapted for this purpose. A current of oxygen was passed over 
one side of the fabric and hydrogen over the other side. The 
oxygen passing into the hydrogen was determined with the inter- 

' Edwards, Preparation and Testing of Hydrogen of High Purity. J. Ind. Eng. Chem. 11. p. 961 ; 1919. 
8 Edwards and Ledig, "Significance of Oxygen in Balloon Gas," Aviation and Aeronautical Eng., 6* 
p. 325: 1919. 



Edwards T 
Pickerings 



Permeability of Rubber to Gases 



349 



ferometer using hydrogen from the same source as the standard 
of comparison. The oxygen was analyzed volume trically, using 
a bturette with a constricted portion in which the unabsorbed 
residue could be measured quite accurately. Five analyses showed 
99-50, 99-53. 99-55. 99-55. and 99.55 per cent oxygen. The results 
were corrected to a partial pressure of 100 per cent, using the 
value 99.55 as the existing partial pressure of oxygen. 

The results of a series of tests are shown in Table 7. The aver- 
age ratio of permeabilities, oxygen to hydrogen, is about 0.45. 
For this ratio Graham found the value 0.466 and Dewar's » curves 
show a value of 0.500 at 25° C. 

TABLE 7.— Permeability of Rubber to Oxygen and Hydrogen 



Fabric No. 


Permeability 
to ozygen 


Permeability 
to hydrogen 


Ratio of per- 
meabilities, 
oiygen to hy- 
drogen 


50313 


Liters per m' 
per 24 hours 

5.09 

5.16 

4.84 

4.97 

4.82 

5.30 


Lifers oer m» 
per 24 hours 

11.83 
11.91 
10.81 
10.77 
11.06 
11.52 


0.430 




.433 


50313 


.448 


50313 -. 


.461 


50313 


.435 


50313 


.460 







Average ratio of permeabilities, oxygen to hydrogen, 0.445. 

3. PERMEABILITY OF RUBBER TO NITROGEN 

The permeability of rubber to nitrogen was determined in the 
same way as the permeability to oxygen except that nitrogen was 
used in place of oxygen. The results of these experiments are 
given in Table 8; the average ratio of permeabilities, nitrogen to 
hydrogen, is 0.16. Graham " gives the value 0.18 and Dewar's " 
value is 0.12 (at 15° C). 

TABLE 8. — Permeability of Rubber to Nitrogen and Hydrogen 



Fabric No. 


Permeability 
to nitrogen 


Permeability 
to hydrogen 


Ratio of per- 
meabilities, 
nitrogen to hy- 
drogen 


50313 


Liters per m^ 
per 24 hours 

1.48 
1.53 
1.45 
1.38 
1.51 
1.27 
1.44 
1.52 


Liters per m^ 
per 24 hours 

8.77 

9.47 

9.14 

9.10 

8.76 

9.08 

9.41 

8.73 


0.169 


50313 


.162 




.159 


50313 


.152 




.172 


50313 


.140 




.153 


50313 


.174 







Average ratio of permeabilities, nitrogen to hydrogen, 0.160. 



s Proc. Roy. Inst.. SI, p. 813; 1915. 



^ Loc. cit. 



'* Loc. cit. 



350 Scientific Papers of the Bureau of Standards [voi. i6 

4. PERMEABILITY OF RUBBER TO ARGON 

No experiments with argon were made in the course of the 
present work because a satisfactory sample of argon was not 
available. For the sake of completeness reference will be made 
to the work of Dewar '^ and of Rayleigh " with argon. Dewar 
found the ratio of the permeabilities to argon and to hydrogen 

to be I — ^ j = o.23 at 15° C. Rayleigh found that in a sample of 

"air" which had diffused through rubber there was 1.93 per cent 
argon in the nitrogen after removing the oxygen. Atmospheric 

nitrogen contains ( " j = i .23 per cent argon. He therefore con- 
cluded that rubber was somewhat more permeable to argon than 
to nitrogen. The ratio of permeabilities, argon to nitrogen, cal- 
culated from his original data, is 1.6. Using the value we have 
found for the ratio nitrogen to hydrogen, the ratio argon to hydro- 
gen would be 0.26. 

5. PERMEABILITY OF RUBBER TO AIR 

The permeability of rubber to air can be calculated from its 
permeability to oxygen and nitrogen by means of the proportion- 
ality between permeability and partial pressure. According to 
Sir William Ramsay the composition of air is as follows: Nitrogen, 
78.12 per cent; oxygen, 20.94 P^r cent; and argon, 0.94 per cent. 
The permeability of rubber to air (referred to hydrogen) would 
then be 

P = (0.7812 X0.16) -1- (0.2094X0.45) -f (0.0094X0.26) =0.22. 

In confirmation of this value, the permeability to air was de- 
termined directly by the same method used in the case of oxygen 
and nitrogen. The refractivity of the air which had diffused 
through the rubber was calculated from the composition as de- 
termined by typical analyses. The influence of any probable 
variation in composition is negligible. The results are given in 
Table 9. Though the data are few in munber and not very con- 
cordant, the average ratio, 0.23, is in close agreement with the 
value (0.22) which was just calculated. 

*2 Loc. cit. *^ Phil., Mag., 49, p. 220; 1900. 



Edwards "I 
Pickeringl 



Permeability of Rubber to Gases 



351 





TABLE 9.— Penneability of Rubb 


er to Air and Hydrogen 




Fabric No. 


Permeability 
to air 


Penneability 
to hydrogen 


Ratio ol 

permeabilities, 

air to 

hydrogen 




Liters per m 2 
per 24 liours 

2.21 
2.14 
1.97 


Liters per m 2 
per 24 hours 

9.45 
8.73 
9.40 


0.234 


50313 


.245 


50313 


.210 







Average ratio of permeabilities, air to hydrogen, 0.230. 

The composition of air which has diffused through rubber is a 
matter of interest. This may be calculated from the permeability 
to nitrogen, oxygen, and argon and their partial pressures. The 
composition thus calculated is as follows: 

Per cent 

Nitrogen 56.8 

Oxygen 42.3 

Argon .9 



Graham found as much as 41 .6 per cent oxygen in air which had 
diffused through rubber; Edwards and Ledig ^^ found 41 per cent. 

These facts are of obvious practical importance in many in- 
stances. 

6. PERMEABILITY OF RUBBER TO CARBON DIOXIDE 

In determining the permeability to carbon dioxide, the regular 
method with the interferometer was employed. The carbon 
dioxide was generated from marble and hydrochloric acid and 
passed over anhydrous sodium carbonate and calcium chloride. 
Volumetric analysis showed the presence of 99.9 per cent carbon 
dioxide. The results of a series of these tests are shown in Table 
10. Each permeability value recorded in the table is the average 
of 3 to 6 separate observations on the same test piece. 

TABLE 10. — Permeability of Rubber to Carbon Dioxide and Hydrogen 



Fabric No. 



Permeability 
to carbon 
dioxide 



Permeability 
to hydrogen 



50313 

50313 

Dental dam . 

26293 

26293 



Liters per m ^ 
per 24 hours 
27.3 
28.6 
42.4 
27.0 
26.6 



Liters per m ^ 
per 24 hours 
9.6 
9.7 
14.0 
9.2 
9.5 



Ratio ot 

permeabilities, 

carbon 

dioxide to 

hydrogen 



2.84 
2.95 
3.03 
2.94 
2.80 



Average ratio of permeabilities, carbon dioxide to hydrogen, 2.91. 



" Edwards and Ledig, Significance of Oxygen in Balloon Gas, Aviation and Aeronautical Eng., 6, 
p. 32s; 1919. 



352 Scientific Papers of the Bureau of Standards [Voi.i6 

The values obtained for the permeability of fabric No. 50313 to 
hydrogen and carbon dioxide in the partial pressure experiments 
recorded in a previous section may be used to obtain a value for 
this ratio, even though the determinations with hydrogen and 
carbon dioxide were not made with the same test pieces. The 
average of seven determinations with h3^drogen was 9.71 liters 
and the average of seven determinations with carbon dioxide was 
27.86; the ratio is 2.87, which is in substantial agreement with the 
ratio 2.91 found in Table 10. It may be concluded that rubber 
is approximately 2.9 times as permeable to carbon dioxide as to 
hydrogen. 

Values foimd by other experimenters for this ratio are of interest. 
Graham'^ gives 2.47 as the ratio (temperature not specified). 
Kayser ^^ gives equations for the variation of permeability with 
temperature; the ratio of the permeabilities of rubber to carbon 
dioxide and hydrogen at 25° C as calculated from these equations 
is 2.48. Dewar," using thin films of rubber under tension and 
having a thickness of about o.oi mm, found a value of 2.5 for 
this ratio at 15° C. In another series of experiments at " ordinary 
temperatures" Dewar ^'^ found for the same film a relative rate 

of I ^-^ J = 2.8. All of these values are lower than those found 

\8-4 / 
in the present work. It should be noted, however, that all three 
experimenters used a volume or pressure method for determining 
the gas penetrating the rubber. Furthermore, the information 
obtainable from the published articles is insufficient to enable one 
to say whether or not the data are on a basis strictly comparable 
with ours. It is certain the results were not obtained under the 
conditions maintained in the present work; that is, an equilibritun 
condition with a continuous stream of pure gas over one side of 
the fabric and a stream of dry air over the other side. There is 
also a constant difference of pressure between the two sides of 
the rubber equivalent to 30 mm of water; the total pressure on 
the air side is 760 mm of mercury. These conditions more nearly 
simulate the conditions of use than those employed in the tests 
just discussed. 

7. PERMEABILITY OF ROBBER TO HELIUM 

A knowledge of the permeability of rubber to heliimi is of great 
importance at the present time, because of the recent develop- 
ment by the United States Government of a supply of helium for 

'sphil. Mas., 82. p. 401; 1866. " Proc. Roy. Inst., 21, p. 813*; 1915. 

'« Wied. Ann.. 43, p. 544; 1891. " Proc. Roy. Inst., 21. p. 559; igis- 



ptk^^] Permeability of Rubber to Gases 353 

filling airships. The investigation at the Bureau of Standards of 
the permeability of balloon fabrics to helium was made for the 
Bureau of Steam Engineering of the Navy Department. The 
helimn employed was furnished by that bmreau and was con- 
tained in a steel cylinder under 1,800 poimds pressure. Its 
composition as determined by our analysis was as follows: 

Per cent 

Helium 94. 6 

Nitrogen 5. 2 

Methane o. 2 

100. o 
The methane in the gas was determined by combustion with 
oxygen. The density of the gas was then determined with the 
Edwards gas-density balance and the composition calculated on 
the assumption that the residue was nitrogen and heliimi. 
Oxygen was tested for and shown to be absent. To confirm the 
analysis, a direct determination of nitrogen was made by absorp- 
tion with metallic calcitun. This method showed approximately 
5 per cent nitrogen. When the gas was examined spectroscopi- 
cally, neon and argon were found to be either absent or present in 
such small amoimts as to be masked by the other gases present. 
Accordingly, it is thought that no appreciable error was intro- 
duced by the assumption that the residue consisted of helium and 
nitrogen. The refractivity of the mixture as determined with a 
Zeiss-Rayleigh interferometer indicated an amount of helium 
within 0.3 per cent of the value shown by the above analysis. 

The amoimt of helium penetrating the fabric was determined 
with the interferometer in the usual way. Because of the large 
difference in the refr activities of air and helitun (2917 — 342) 
X io~^ the interferometer fvu-nished a very sensitive means of 
determining helium. Each scale division indicated about 0.004 
per cent helium in air; the total amount present could be deter- 
mined with that precision. 

The observed permeability obtained with the gas containing 
94.6 per cent helium was corrected to the standard condition; that 
is, a difference in partial pressure of 100 per cent heliimi on the 
two sides of the fabric. This was done by multiplying the 

100 
observed permeability by the ratio -— - where x is the per- 
centage of helium (usually about 0.4 per cent) on the "air side" 
of the fabric. 



354 



Scientific Papers of the Bureau of Standards 



[Vol. i6 



In Table 1 1 are given the permeabilities to helium and hydrogen 
of a number of samples of different fabrics. The fabrics tested 
are from three different manufacturers and include both envelope 
and ballonet fabrics. Although there was considerable variation 
in the relative permeabilities, this variation could not be entirely 
ascribed to experimental error. It seems probable that part of 
this variation is due to differences in the relative permeabilities 
of different fabrics to these gases. The average ratio of 0.65 
appears satisfactory for present purposes. 

TABLE 11. — Permeability of Rubber to Helium and Hydrogen 



Fabric No. 



Observed 

permeability 

to helitun 



Permeability 

corrected for 

100 per cent 

belium 



Observed 
permeability 
to bydrogen 



Ratio of 

permeabilities^ 

helium to 

bydrogen 



27145.. 
45847., 
45835., 
35838. , 
36827. 
36827., 
36827. 
36827. 
36827., 
36827. 
36827. 
36827. 
36293. 
24579. 



Liters per m^ 
pel 24 bouts 

9.6 

9.8 

7.6 
7.4 
6.4 
6,7 
6.5 
6.2 
6.6 
6.1 
6.3 
6.5 
5.7 
4.8 



Liters per m^ 

per 24 bours 

10.2 

10.3 

8.0 

7.9 

6.8 

7.0 

6.9 

6.6 

7.0 

6.4 

6.7 

6.9 

6.1 

5.1 



Liters per m^ 
per 24 hours 

16.1 

15.7 

14.0 

13.7 

10.0 

10.6 

10.7 

10.0 

10.6 

9.5 

9.3 

9.4 

9.8 

8.7 



0.63 
.66 
.57 
.58 
.68 
.66 
.64 
.66 
.66 
.67 
.72 
.73 
.62 
.59 



Average ratio of permeabilities, belium to hydrogen, 0.65. 

The other values for this ratio which appear in the literature 
are those of Dewar " and Barr.^ Dewar found a ratio of 



m- 



31. No information is given regarding the helium 



used. Barr estimated the permeability to helium to be about 
two-thirds of the permeability to hydrogen, a value which is in 
agreement with ours. 

8. PERMEABILITY OF RUBBER TO AMMONIA 

Ammonia has been considered as a filling gas for balloons. 

Its specific gravity is only 0.596, and it offers advantages from 
the standpoint of freedom from fire hazard and the fact that it 
can be transported in the liquid form. However, the fact that 
rubber is quite permeable to ammonia would necessitate the use 
of a different tjrpe of fabric for the balloon envelope. 



»Loc. dt. 



"Barr British Advisory Comm. for Aeronautics, 1915. 



Edwards "| 
Ptcker-.ttffj 



Permeability of Rubber to Gases 



355 



In determining the permeability of rubber to ammonia it was 
necessary to use a special permeability cell made of steel, which 
would be unattacked by the ammonia. All connecting tubes 
coming in contact with the ammonia were either steel or glass. 
The ammonia which passed through the fabric into the air stream 
was absorbed in a measured voliune of tenth-normal sulphuric 
acid and the excess acid determined by titration. Two small 
wash bottles were always used in series, but never more than a 
negligible amoimt of ammonia escaped absorption in the first 
wash bottle. Two sets of wash bottles were used, and they were 
connected to the cell alternately through a three-way cock. 
They were attached to the system by a mercury seal so that they 
could be easily detached. Each value recorded in the table is 
the average of a number of obser\rations, each covering a half- 
hom- period. 

The ammonia was taken from a small steel cylinder, which had 
been evacuated to a very low pressure before filling with liquid 
ammonia. The gas can be considered to be free from air and 
water vapor; in fact, the total amount of impurity in this sample, 
which was carefully purified by fractionation, was shown by 
tests of C. S. Taylor, of this Bureau, to be less than i part per 
loo coo. The results of a series of experiments are given in Table 
12. It was noted that it took considerable time to remove all 
the ammonia from the rubber, so that it was necessary to deter- 
mine the permeability to hydrogen first in each case. The average 
ratio of the permeability to ammonia and hydrogen is probably 
very close to 8. 

TABLE 12. — Permeability of Rubber to Ammonia and Hydrogen 



Fabric No. 


Permeability to 
ammonia 


Permeability to 
hydrogen 


Ratio of 

permeabilities, 

ammonia to 

hydrogen 


50313 


Liters per m^ per 24 
hours 

71.9 

59.3 

> a 
61. l| 

1 ;;:::!"- 


Liters per m2 per 24 
hours 

9.0 

9.1 
10. 

1 h::i - 


7.99 


50313 


8.02 


50313 


8.04 




7.93 







Average ratio of permeabilities, ammonia to hydrogen, 8. 



a These two results which were obtained on two succeeding days indicated that some change had oc- 
curred in the sample, and they are omitted from the average. 



356 



Scientific Papers of the Bureau of Standards 



[Vol.16 



9. PERMEABILITY OF RUBBER TO ETHYL CHLORIDE 

The permeability of rubber to ethyl chloride (C2H5CI) is prin- 
cipally of interest because of the high value found and its 
relation to their mutual solubility. An interferometer of the 
portable type was used for determining the percentage of ethyl 
chloride passing through the fabric into the air stream. The 
interferometer was calibrated by the method previously referred 
to; for the purposes of this calibration the refractivity of ethyl- 
chloride vapor was calculated from values for the refractive index 
of the saturated vapor as recently determined at this Bureau. In 
making this calculation the refractivity of ethyl-chloride vapor at 
the partial pressures at which it was measured (4 to 5 per cent) 
was estimated from the density, which was calculated by means 
of Berthelot's equation of state. In doing this it was assumed 
that the partial pressure of the ethyl-chloride vapor in air followed 
Dalton's law. 

The ethyl chloride was contained in a glass flask fitted with a 
steel valve, and the vapor could be readily withdrawn under its 
own pressure. It was prepared by C. S. Taylor, of this Bureau, 
from very pure materials and further purified by fractionation. 
The total impurity in the vapor phase was undoubtedly less than 
I part in 10 000, a ptuity far beyond that required for the present 

work. 

TABLE 13. — Permeability to Ethyl Chloride and to Hydrogen 



Fabric Nc. 



PenneabilKy 
to etbyl 
chloride 



Permeability 
to hydrogen 



Ratio of 

permeabilities, 
ethyl chloride 
to hydrogen 



50313. 
50313. 
50313. 
50313. 



Liters per m- 
per 24 hours 

1717 

1851 

1813 

1763 



Liters per m^ 
per 24 hours 

8.80 

9.76 

8.31 

9.44 



195 
190 
218 
187 



Average rates of permeabilities, ethyl chloride to hydrogen, 198. 

The permeability of rubber to ethyl chloride as shown by these 
few tests is approximately 190 to 200 times its permeability to 
hydrogen. 

10. PERMEABILITY OF RUBBER TO METHYL CHLORIDE 
For the purpose of comparison with ethyl chloride, the permea- 
bility of rubber to methyl chloride (CH3CI) was also determined. 
The interferometer was employed in estimating the percentage of 
methyl chloride in the same manner as with ethyl chloride. The 
only available data on the refractivity of methyl chloride are 
those of Mascart {N-d, 0° C, 760 mm = i .000870) ; ^^ the calibration 

21 From Landolt-Bornstein Phys. Tabellen. 



Edwards I 
PickerinoJ 



Permeability of Rubber to Gases 



357 



is based on this value. The sample of methyl chloride used had 
been carefully purified by fractionation by C. S. Taylor. Three 
tests were also made with a sample of methyl chloride, the purity 
of which was unknown. These gave a ratio of permeabilities, 
methyl chloride to hydrogen, of 16.8, 17.6, and 17.8, but they are 
not included in the average. The other results are given in Table 
14. The permeability of rubber to methyl chloride is approxi- 
mately 18.5 times its permeability to hydrogen. 

TABLE 14.— Permeability of Rubber to Methyl Chloride and Hydrogen 



Fabric No. 


Permeability 
lo methyl 
chloride 


Permeability 
to hydrogen 


Ratio of 

permeabilities. 

methyl chloride 

to hydrogen 


50313 


Liters per m- 
per 24 hours 

173.8 

185.8 

174.8 

180.3 


Liters per m^ 
per 24 hours 

9.43 

9.58 

9.68 

9.86 




50313 




50313 


18 1 


50313 









Average rates of permeabilities, methyl chloride to hydrogen, 18.5. 

11. PERMEABILITY OF RUBBER TO WATER VAPOR 

The permeability of rubber to water vapor is interesting for a 
number of reasons. In view of the popular conception of rubber 
as a "waterproof" material, it might be thought that it was quite 
impermeable to water vapor, whereas the opposite is true — its 
permeability is relatively high. This fact is of great importance 
in many instances where rubber is used as a gas container; such 
as, for example, the use of rubber tubing in chemical and physical 
work. The use of rubber connections in any apparatus where 
the water content of the gas is important may introduce more or 
less serious errors. 

The high permeability of rubber to water vapor renders its 
determination rather difficult. The first method employed in its 
determination was to pass a current of air saturated with water 
vapor at a temperatvire slightly below 25° C over the fabric, 
which was maintained in the cell at 25° C. A stream of air, pre- 
viously dried over phosphorus pentoxide, was passed over the 
other face of the fabric and thence through an absorption tube 
filled with phosphorus pentoxide in which the water vapor could 
be absorbed and weighed. The results were very erratic, prob- 
ably because of the low partial pressure of the water vapor in the 
air (about 3 per cent) and the large effect on the difference in 
partial pressure produced by small variations in the rate of passage 
of the dry air. The results, however, are confirmatory of those 
secured by the following method: 



358 



Scientific Papers of the Bureau of Standards 



[Vol. i6 



A shallow, crystallizing dish, 8 cm in diameter, was partly 
filled with phosphorus pentoxide and the top closed by a sheet of 
rubber, such as dental dam, which was fastened at the edge with 
rubber cement. The dish was then placed in an atmosphere 
saturated with water vapor and the rate of increase in weight 
determined. The results are shown in Table 15; obviously they 
only give an approximate figure, and no claim of accuracy is made 
for them. Lack of time prevented carrying this phase of the 
work farther. In connection with this table and the succeeding 
one, attention should be called to the fact that the permeability 
to water vapor is calculated for the assumed case of a difference in 
partial pressure of water vapor of 760 mm. This is done to make 
the results comparable with the hydrogen value. In any test 
the partial pressure of water vapor was about 20 mm. 

TABLE IS.— Penneability of Rubber to Water Vapor and Hydrogen 

[All saturated with water vapor in contact with rubber) 



Sample No. 


Permeability 

to water vapor 

(100 per cent 

partial 

pressure) 


Permeability 
to hydrogen 


Katio of per- 
meabilities, 
water vapor to 
hydrogen 


A 17_TIiidrn(.<;«, n.lR mm 


Liters per m' 
per 24 hours 

951 

969 
1270 
1001 
1108 

978 
1130 
1174 

975 

920 
1021 

875 

970 

890 
1030 
1019 
1262 
1075 


Liters per m- 
per 24 hours 


















































































































1034 


22.0 


47.0 




A lQ_ThirUii«», n.7A mm 


905 
905 
726 
1118 
860 
765 
905 
930 






















































Average 


889 


14.3 


62.0 





Edwards 1 
PickerinQj 



Permeability of Rubber to Gases 



359 



A few experiments were also made with liquid water in con- 
tact with the rubber film. In these tests instead of cementing the 
rubber to the dish containing the phosphorus pentoxide, the 
rubber was cemented to the top of another exactly similar dish 
from which the bottom had been removed. The edges of both 
dishes were grotmd plane. The dish with the rubber film across 
the bottom was partially filled with water and placed on top of 
the dish containing the phosphorus pentoxide. When it was 
desired to weigh the lower dish, the upper dish was replaced by a 
watch glass. The results of these tests are shown in Table i6. 
In calculating the results the partial pressure of water vapor used 
was that corresponding to the temperature of the water in contact 
with the rubber. 



TABLE 16.— Permeability of Rubber to Water Vapor 
[Liquid in contact with rubber] 



Sample No. 


Permeability 
to water vapor 


Permeability 
to hydrogen 


Ratio of per- 
meabilities, 
water vapor to 
hydrogen 




Liters per m^ 
per 24 hours 

1526 

1700 

1846 

1918 


Liters per m: 
per 24 hours 






























1748 


18.4 


95.0 






A-19 — Thickness, 0.25 nun 


1510 
1752 
1740 
1581 
1638 
1712 
1562 


















































1642 


14.3 


115 







According to these few tests, the permeability of rubber to 
water vapor is about 50 times the permeability to hydrogen when 
saturated air is in contact with the rubber and about 100 times 
when liquid water is in contact with the rubber. In these methods 
diffusion processes were depended on to bring the water vapor 
into contact with the rubber and from the rubber to the phos- 
phorus pentoxide. This factor should tend to give low values. 
The accidental errors of handling and weighing would probably be 
in the opposite direction. 



360 Scientific Papers of the Bureau of Standards [vei. 16 

Dewar in the work previously cited found for very thin rubber 
films in contact with liquid water a value which is 29.1 times the 
rate for hydrogen, both being measured at 15° C. Dewar foimd 
under similar conditions with ethyl alcohol in contact with the 
rubber a ratio of 25.9 which is lower than the value for water. 

According to Kahlenberg,^^ if water and alcohol are separated 
by a rubber film, the alcohol passes through into the water faster 
than the water passes into the alcohol which indicates a higher 
permeability to alcohol than to water. If both investigators are 
correct, it is an interesting case of the modification of perme- 
ability by another substance. 

The present authors ^' fotmd that saturating rubber with car- 
bon dioxide did not change its permeability to hydrogen, but 
then, of course, the total dissolved gas was lower than in the 
case of either water or alcohol. Further data on these effects 
would be of interest. 

VII. THEORY OF PERMEABILITY 

One object of this investigation was to estabUsh, if possible, a 
quantitative relationship between the permeability of a film of 
rubber to any particular gas and the various factors on which it 
is dependent. Only the part of the program detailed in the pre- 
ceding pages was completed, however, before it became necessary 
to discontinue the work. 

A simple and satisfactory picture of the process is one of dynamic 
equilibrimn, in which the gas is dissolved at one side of the rub- 
ber at a rate proportional to its solubility and partial pressure 
and diffuses through the rubber where it evaporates from the other 
side. The same process takes place in the opposite direction, so 
that the net transference of gas is proportional to the difference 
in the partial pressures at the two faces of the rubber. Because 
of the lack of data it is not feasible to analyze the relations between 
solubility and rate of diffusion through the rubber. The perme- 
ability in every case investigated increases rapidly with increase 
of temperature. According to Kayser,-* the solubility of both 
carbon dioxide and hydrogen decreases with increase of tempera- 
ture. If this be true, there must be a rapid decrease in the inter- 
nal resistance of the rubber to the passage of the gas, because 
the ordinary temperatm-e coefficient of gaseous diffusion is imable 
alone to account for the facts. 

22 J. Phys. Chem., 10, p. 141: 1906. 
23J.Ind.Eng.Chem., 11, p. 966; 19J9. 
2*Wied. Ann. 43, p. 544; 1S91. 



pi^m^tl Permeability of Rubber to Gases 361 

A rough parallel, with notable exceptions, may be Arawn 
between the permeability of rubber to different gases and to 
the boiling points of the gases. In general, the higher the boil- 
ing point of the gas the greater the rate at which it penetrates 
rubber. The specific chemical characteristics of the gas and of 
the rubber colloid determine, however, the solubility, rate of 
penetration, etc., and not enough is known of them at the present 
time to warrant further speculation. There are, however, many 
interesting fields of investigation opened by this work, and the 
results should be extremely useful in the many cases where the 
behavior of rubber in contact with gases is concerned. 

VIII. SUMMARY 

Certain of the factors which determine the permeability of 
rubber to gases have been investigated and the relative rates of 
penetration of a number of gases determined. The major findings 
may be summarized as follows : 

1 . The permeability of rubber compoimds varies with the com- 
position, as would be expected. The aging of rubber films is 
accompanied by a decrease in permeability; a similar decrease 
may be affected by overvulcanization. The rubber which shows 
a very low permeability for these reasons is usually very much 
deteriorated and frequently brittle, so that it is a disadvantage 
from the standpoint of gas tightness. 

2. The permeability to any gas is found to be directly pro- 
portional to its partial pressure, provided the total pressure is 
constant. The variation of permeability with total pressure 
depends on the thickness of the rubber, the way in which it 
is supported, etc. 

3. The permeability to hydrogen is inversely proportional to 
the thickness of the rubber. No other gas was tested in this 
respect. 

4. The specific permeability to hydrogen at 25° C of vulcanized 
rubber similar to the grade known as dental dam is about 20 X 10"° 
cc per minute. This value varies somewhat with the age and 
chemical characteristics of the rubber. 

5. The temperature coefficient of permeability is quite high. 
For example, in the tests at 100° C the permeability to carbon 
dioxide or helium was about 1 7 times the rate at 0° C ; the per- 
meability to hydrogen was about 22 times as great at 100° as 
at 0° C. 



362 



Scientific Papers of the Bureau of Standards 



iVol. iDJ 



6. The relative permeability of rubber to some common gases 
is shown in the following summary : 

TABLE 17.— Relative Penneability of Rubber 



Gas 



Relative 
penneability* 
hydTogeD= 1 



Gas 



Relative 
permeability, 
hydrogen= 1 



Nitrogen. 

Air 

Argon... 
Ojygen.. 
Helium.. 



0.16 
.22 
.26 
.45 
.65 



Hydrogen 

Carbon dioxide. 

Ammonia 

Methyl chloride 
Ethyl chloride.. 



1.00 
2.9 
8.0 
18.5 
200.0 



7. The permeability of rubber to water vapor is high — approxi- 
mately 50 times the permeability to hydrogen. This value not 
having been determined with any precision is not included in 
the table above. 

Special acknowledgment is due the Goodyear Tire & Rubber Co. 
for the samples of rubber and fabric furnished to us in the course 
of this work. Many of these were especially constructed to meet 
otir specifications. Their enthusiastic cooperation was of great 
assistance. 

Washington, February 25, 1920. 



mM:^x-my->:- 



